Electrode for electrochemical reactions

ABSTRACT

An electrode for electrochemical reactions comprising a substrate of a film forming or barrier metal covered with a cobaltite of at least two rare earth metals, one of the rare earth metals having a high atomic number and the other having a lower atomic number.

United States Patent. [191 Bouy etal. Nov. 4, 1975 ELECTRODE FORELECTROCHEMICAL [56] Reference; Cited REACTIONS UNITEDSTATES PATENTS[75] Inventors: Pierre Bouy, Enghien-les-Bains; Guy 3,329,594 7/1967Anthony et al. 204/95 Cheradame, Pont-de-Claix, both f 3,801,490 4/1974Welch 204/290 F France 3,804,740 4/1974 Welch 204/290 R [73] -Assignee:Rhone-Progil, Courbevoie, France FOREIGN PATENTS OR APPLICATIONS FiledJu y 5 1 4 7,204,743 6/1973 Netherlands 204/290 F [21] Appl. N0.:486,051 Primary Examiner-F. C. Edmundson [30] Foreign ApplicationPriority Data [57] ABSTMCT July 20 I 973 France 73 26694 An electrodefor electrochemical reactlons comprising a substrate of a film formingor barrier metal covered with a cobaltite of at least two rare earthmetals, one '3 ggiff of the rare earth metals having a high atomicnumber [58] Field 0 204/290 1: 290 F 291 and the having a ammi" number"5 Claims, No Drawings ELECTRODE FOR ELECTROCHEMICAL REACTIONS BACKGROUNDOF THE INVENTION The present invention concerns a new electrode whichcan be used in electrolytic cells serving for the production ofchlorine, caustic soda or chlorates. The cells serving for theproduction of chlorine or caustic soda are either diaphragm cells ormercury cells. The chlorates are produced in a cell whose structure issimilar to that of the diaphragm cells but which nevertheless has nodiaphragm.

The electrodes previously generally employed as anodes in electrolyticcells were frequently made of graphite. Their use has always entailedcertain disadvantages resulting from their wear which causes an increasein the voltage necessary for the proper operation of the electrolysiscell as the result of the wear which increases in the distance betweenanodes and cathodes and the contamination of the electrolyte.

More recently it has been attempted to develop anodes from a metalhaving good resistance to corrosion by the electrolyte which metal iscovered with an electrochemically active precious metal, the resultingcomposite then being subjected to a treatment which favors activation.These anodes are dimensionally stable and do not have theabove-mentioned drawbacks. For anodes of this type it has been proposedto employ a core of zirconium, zirconium-titanium alloy, tantalum orniobium covered with platinum. There has also been proposed an anode oftitanium covered with platinum. Titanium, like the other core metalsmentioned above, being a film forming or barrier metal capable offorming a film or barrier layer of oxide in the electrolysis solutionsto protect its surface from corrosion at the places where the platinumis porous.

Also, electrodes have been produced of one of these film forming orbarrier metals or alloys capable of forming a film or barrier layer,covered with an oxide of precious metal or with mixtures of oxides ofprecious and non-precious metals.

As an electrode covering or coating there has also been proposed anelectrolytic deposit of cobalt oxide, the electrocatalytic properties ofwhich are very close to those of the precious metals, their alloys ortheir compounds. It is also known that deposits of saline oxide, cobaltoxide (C 0 have properties very close to those of the precious metals.However, none of these compounds of cobalt can be used in solid form oras deposit in industrial practice as a result of the lack of stabilityof their electrocatalytic properties. As a matter of fact, thesecompounds when used as anodes, rapidly become electrically insulatingand oppose the passage of the current, thus producing a resistance whichleads to prohibitive overvoltages.

It has recently been suggested that these drawbacks could be avoided bymeans of an electrode formed of a substrate or titanium or other similarfilm forming or barrier metal covered with a thin film of anelectroconductive coating, of a metal of the platinum group, forinstance, on which an outer layer or surface of perovskite is applied.The perovskite is an oxygenated compound of two different metals whichis well known in the literature and may be represented by the empiricalformula:

. a special X-ray diffraction diagram.

These cobaltites have a relatively high electric conductivity whichvaries with the temperature, the rare earth metal playing an importantrole in the mechanism of conduction.

The electrocatalytic power of these cobaltite compounds is notnecessarily related to the perovskite structure, since there arenumerous compounds having this structure, such as, for instance, LaCrOlanthanum chromite, which are without it. However, it is necessary inthe case of the rare earth cobaltites to obtain the perovskite structurewhich alone seems to withstand corrosion in slightly acid medium. It hasbeen noted that this corrosion is smaller the more acid the character ofthe rare earth used. The compound LaCoO lanthanum cobaltite, forinstance, although having remarkable electrocatalytic properties, isentirely unsuitable to constitute an anode for an electrolysis cell as aresult of the ease with which it passes into solution in slightly acidchlorinated medium. This defect decreases when the lanthanum is replacedby a rare earth of higher atomic number. One succeeds in this way inconsiderably improving the resistance to chemical and electrochemicalcorrosion by using rare earths of higher and higher atomic number, aswill be shown below:

The compounds LaCoO PrCoO NdCoO and GdCoO are prepared from an intimatemixture of the oxides of the stated. elements, which is calcined at1200C. for 15 hours. The series of compounds thus prepared is analyzedby X-ray diffraction and is found in each case to be solely of theperovskite structure. The chemical resistivity in acid medium of thesemixed oxides is then measured as follows:

To 1 gram of the powder of each compound there are added 200 ml. of O.IN hydrochloric acid. The attack is allowed to continue for 1 hour inthe cold. After filtration, the cobalt and the rare earth present in thefiltrate are determined. The following table sets forth the corrosion ofthe compounds in terms of percentage, that is to say, the ratio of thetotal mass of the metal elements present in the solution to the totalmass of the metal elements present in 1 gram of cobaltite.

Corrosion.

LaCoO 35% PrCoO 9.6% NdCoO 5.7% GdCoO 4.3%

LaCoO PrCoO NdCoO GdCoO about about about about Time 1 hr. 30 hr. 400hr. 500 hr.

There is thus noted the good correlation between the electrolysis lifeand the corrosion in acid medium.

However, one is limited in the use of the heavy rare earth metals aselectrodes in electrolysis cells by the tendency which these rare earthshave to give in whole or in part a mixed solid phase, Co (TR) ,O whichis more or less rich in cobalt and known by crystallographs under thephase designation CTl O A range of existence of the differentcrystalline phases has been established and is described, for instance,on page of the book by F. S. Galasso, Structure Properties ofPerovskite-Type Compounds, Pergamon Press, 1969. This limitation is verydisturbing, since the cobalt oxide phase, rare earth metal oxide of thestructure CTI O being readily soluble in acids, is unsuitable for thedesired use in electrolysis. This particular behavior of the rare earthsof high atomic number is explained by crystallographic considerationsutilizing ionic rays.

It is, accordingly, an object of the present invention to provideelectrodes for electrochemical reactions which do not have theshortcomings of the prior art.

It is also an object of the present invention to provide an electrodefor an electrolytic cell which employs a cobaltite of perovskitestructure, which electrode has improved properties.

It is a further object of the present invention to provide electrodesfor electrolytic cells, which electrodes have excellent resistance tocorrosion.

Further objects will be apparent to those skilled in the art from thepresent description.

GENERAL DESCRIPTION OF THE INVENTION It has now surprisingly beendiscovered that these drawbacks of the prior art can be eliminated bymeans of employing new rare-earth or rare earth metal cobaltitecompounds comprising at least two rare earth metals, one or more ofthese rare earth metals having a high atomic number of at least about 65and not resulting in a compound of perovskite structure when they arecombined alone with the cobalt. Another of the rare earth metals havingan atomic number below about 65. This new rare-earth cobaltite compoundhas a special X-ray diffraction pattern and a characteristic perovskitestructure. This diffraction pattern and structure are fully described inthe literature. For instance, in Chapter 5 of the book, DiffractionProcedures, by Klug and Alexander, John Wiley and Sons (1954), see pages235 to 318.

The new electrodes in accordance with the invention comprise a substrateof a film forming or barrier metal covered with a cobaltite compounddescribed above which forms the surface of the electrode. This compoundhas the general formula in which Ln represents a rare earth metal ofhigh atomic number, such as at least about 65, Ln a rare 4 earth metalof lower atomic number, such as below about 65, and x is a numberbetween 0.001 and 0.999, and preferably between about 0.05 and 0.3.

The new cobaltite compounds in accordance with the invention have asubstantially higher resistance to acid corrosion than the known rareearth metal cobaltites, while having the same characteristics ofconductivity and the same electrocatalytic properties.

The rare earth metals which can be used are those listed in the PeriodicTable of the Elements. Those of high atomic number comprise terbium,dysprosium, holmium, erbium, thulium, ytterbium and lutetium. The rareearths of lower atomic number comprise lanthanum, cerium, praseodymium,neodymium, samarium, europium and gadolinium.

The substrate, or core, of the electrode is advantageously formed offilm forming or barrier metal, that is to say, of metal forming apassivating layer of oxide which permits the passage of current only inthe direction towards the cathode. These film forming metals are wellknown and include, for example, titanium, tantalum, tungsten, hafnium,zirconium, aluminum, niobium and their alloys. Graphite can also be usedand is intended to be included in the term film forming metal as usedherein. The substrates may be solid pieces or thin, non-perforatedplates. They may also be of perforated plates or metal gauze. Theirshape is desirably that customarily employed for the anodes ofelectrolysis cells.

It has been found that the value of the ionic rays of the component rareearth metals of the cobaltite compound is important, and that it is notpossible to combine merely any rare earth metals in any proportion. Thusif one uses a rare earth having an ionic radius as small as that oferbium, it is necessary to introduce a rather large proportion of a rareearth metal having a rather high ionic radius such as that of neodymium.

Of course, the rare earth cobaltite need not be limited to two rareearths, but may comprise three rare earths or even more, the essentialfactor being the retention of the perovskite structure from one or morerare earth metals leading to this structure with one or more rare earthsnot leading to it.

These new compounds may be prepared like all the other cobaltites orperovskite structure by processes well known to the man skilled in theart. That is to say, thermolyzable organic or inorganic salts, oxides orhydroxides of the different elements are mixed, coprecipitated andcocrystallized. Then after the drying and crushing operations, thepowder obtained, whether or not compacted, is calcined at a temperaturebetween about 900 and about 1500C. for a period of time which may varyfrom 2 hours to 72 hours. In general, the perovskite compounds which canbe used for the electrodes of the invention may be prepared by any ofthe processes described in the literature. For example, by the processdescribed in the journal American Mineralogist, Vol. 39 (l), 1954.

SPECIFIC DISCLOSURE OF THE INVENTION In order to disclose more clearlythe nature of the present invention, the following examples illustratingthe invention are given. It should be understood, however, that this isdone solely by way of example and is intended neither to delineate thescope of the invention nor limit the ambit of the appended claims. Inthe examples which follow, and throughout this specification, thequantities of material are expressed in terms of parts by weight, unlessotherwise specified.

EXAMPLE 1 Compounds are prepared of the general formula Gd e Tb Coo inwhich x is the quantity of Gd ions in the gadolinium cobaltite which arereplaced by terbium ions.

These compounds are prepared from an intimate mixture of gadolinium,terbium and cobalt oxides the quantities of which, as a function of x,are summarized in Table 1 below:

Table l X 0121 0 Tb,O-, Cobalt oxide content (grams) (grams) 71% (grams)The mixtures of oxides are compresed under a pressure of tons into theform of pellets and then 'calcined at 1200C. for hours. The calcinedpellets are then crushed into fine form.

The resulting series of compounds thus prepared is analyzed by X-raydiffraction for identification of the phases. Table 2, below, summarizesthe results obtained:

Table 2 x 0 perovskite structure x 0.05 perovskite structure x 0.1perovskite structure x 0.2 perovskite structure very little C-Tl Ostructure x 0.3 perovskite structure abundant C-Tl 0 structure x 0.5perovskite structure very abundant C-Tl O structure x 1 very littleperovskite structure c-rl o, structure The chemical resistivity in acidmedium of these mixed oxides is then measured as described above. Table3, below, summarizes the results obtained:

Table 3 Gd ,,Tb,CoO: x Corrosion 9999. LIILQN O It is thus noted thatthe compounds of the general formula Gd Tb,CoO have minimum corrosionfor the highest possible quantity of terbium, which leads to the onlytrue perovskite structure, that is to say, for x 6 then crushed untilthe size of the grains is less than 10 microns. The black powder thusobtained has a characteristic X-ray diffraction pattern of theperovskite structure of the cobaltites.

The cobaltite thus prepared is then deposited on a titanium plate of 10mm. width by 30 mm. length and 1 mm. thickness which has been previouslycleaned by sanding, washed with distilled water. and dried.

A suspension of the cobaltite is prepared in the following manner: To 1gram of powder there is added 1 gram of hydrated cobalt nitratehexahydrate, 1 ml. of water and 1 ml. of isopropyl alcohol. The pasteobtained is agitated vigorously until homogeneous suepension isobtained, the agitation being maintained during the production of thedeposit. A layer of the suspension of the cobaltite is applied on thesurface of the titanium plate by brush. After drying for 5 min. in anoven at 100C, the resulting electrode is kept for 10 min. in a furnaceat a temperature of 400C. while it is swept by air. This operation isrepeated 20 times. The amount of product deposited is 40 mg./cm Thedeposit on the electrode consists of cobaltite and 20% cobalt oxide.

The electrode thus prepared is placed in an electrolysis cell for themanufacture of chlorine and caustic soda, in which the electrolyte is asolution of 300 grams per liter of sodium chloride maintained at 80C.and a pH of 4. A current such as to produce an anodic current density of25 amperes per square decimeter is then passed into the cell; the anodicoxidation voltage of the chloride ions is l millivolts when referred toa saturated calomel electrode. After 1000 hours of electrolysis, theanode potential remains unchanged.

EXAMPLE 2 In accordance with the procedure of Example 1, compounds areprepared of the general formula Gd ,Dy CoO from gadolinium, dysprosiumand cobalt oxides, the quantities of which, as a function of x, aresummarized in Table 4, below:

An analysis of the powders by X-ray diffraction confirms the resultsobtained in Example 1 and shows an evolution for x increasing from O to1 of the perovskite structure towards the structure C-T1 O The chemicalresistivity of these mixed oxides in 0.1N hydrochloric acid medium isthen measured, the results being summarized in Table 5, below:

Table 5 9 9. moan-O Corrosion It is thus found that the compounds ofgeneral formula Gd Dy CoO have a minimum corrosion for the highestpossible quantity of dysprosium, which con- 1 fers upon the product theonly completely perovskite structure, that is to say, for x equal 0.1.

An electrode is prepared with a surface of Gd Dy C on a titanium platein accordance with the pro- 1 cedure of Example 1. This electrode isused as electrolysis anode for the manufacture of chlorine. For a brineof 300 grams per liter at 80C. and a pH of 4, there is obtained a strongliberation of chlorine with a current density of 25 amperes per squaredecimeter, under a voltage of 100 millivolts when referred to asaturated calomel electrode. After a prolonged period of electrolysis,the anode potential remains unchanged.

EXAMPLE 3 In accordance with the procedure of Example 1, compounds ofthe general formula Nd( ,Tb CoO are prepared from neodymium, terbium andcobalt oxides the quantities of which, as a function of x, aresummarized in Table 6, below:

Table 6 x Nd O Tb.,O Cobalt oxide content (grams) (grams) 71% (grams) Ananalysis of the powders by X-ray diffraction confirms the resultsobtained in Examples 1 and 2 and shows an evolution for x increasingfrom 0 to 1 of the perovskite structure towards the CTl O structure.

The chemical resistivity of these mixed oxides is then measured in an0.1N hydrochloric acid medium, the results of which are summarized inTable 7, below:

T able '7con1inued It is thus found that the compounds of generalformula Nd Tb CoO present minimum corrosion for the highest possibleamount of terbium, which confers 0 upon the product the only completelyperovskite structure, that is to say, for x equal 0.2.

An electrode is prepared having a surface of Nd Tb CoO on a plate oftitanium by means of an organic or inorganic binder in accordance with aprocedure substantially the same as that of Example 1. This electrode isused as electrolysis anode for the manufacture of chlorine. For a brineof 300 grams per liter at 80C. and a pH of 4, there is obtained a strongliberation of chlorine at a current density of 25 amperes per squaredecimeter under a voltage of 1100 millivolts against a saturated calomelelectrode. After a prolonged time of electrolysis, the anode potentialremains unchanged.

As will be apparent to those skilled in the art from the foregoingdisclosure, cobaltites of other rare earth metals may be employed in theforegoing examples.

The terms and expressions which have been employed are used as terms ofdescription and not of limitation, and there is no intention in the useof such terms and expressions of excluding any equivalents of thefeatures shown and described or portions thereof, but it is recognizedthat various modifications are possible within the scope of theinvention claimed.

What is claimed is:

1. An electrode for electrochemical reactions, comprising a substratecovered with a compound having a perovskite structure, characterized bythe fact that the substrate is of a film forming metal and the compoundof perovskite structure is a cobaltite of rare earths having the generalformula Ln Ln ,CoO in which Ln has an atomic number of at least aboutand Ln has an atomic number below about 65, wherein x is between 0.001and 0.999.

2. An electrode according to claim 1, in which x is between about 0.05and 0.3.

3. An electrode according to claim 1, in which the rare earth of Ln hasa high atomic number and is a member selected from the class consistingof terbium, dysprosium, holmium, erbium, thulium, ytterbium andlutetium.

4. An electrode according to claim 1, in which Ln is a member selectedfrom the class consisting of lanthanum, cerium, praseodymium, neodymium,Samarium, europium and gadolinium.

5. An electrode according to claim 1, in which the film forming metalsubstrate is a member selected from the class consisting of titanium,tantalum, tungsten, hafnium, zirconium, aluminum, niobium and theiralloys.

1. AN ELECTRODE FOR ELECTROCHMECIAL REACTIONS, COMPRISING A SUBSTRATECOVERED WITH A COMPOUND HAVING A PEROVSKITE STRUCTURE, CHARACTERIZED BYTHE FACT THAT THE SUBSTRATE IS OF A FILM FORMING METAL ANDTHE COMPOUNDOF PEROVSKITE STRUCTURE IS A COBALTITE OF RARE EARTHS HAVING THE GENERALFORMULA LNXLN'' (1-X)CO03 IN WHICH LN HAS AN ATOMIC NUMBER OF AT LEASTABOUT 65 AND LN'' HAS AN ATOMIC NUMBER BELOW ABOUT 65, WHEREIN X ISBETWEEN 0.001 AND 0.999.
 2. An electrode according to claim 1, in whichx is between about 0.05 and 0.3.
 3. An electrode according to claim 1,in which the rare earth of Ln has a high atomic number and is a memberselected from the class consisting of terbium, dysprosium, holmium,erbium, thulium, ytterbium and lutetium.
 4. An electrode according toclaim 1, in which Ln'' is a member selected from the class consisting oflanthanum, cerium, praseodymium, neodymium, samarium, europium andgadolinium.
 5. An electrode according to claim 1, in which the filmforming metal substrate is a member selected from the class consistingof titanium, tantalum, tungsten, hafnium, zirconium, aluminum, niobiumand their alloys.